1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing a thin film, using a cluster beam deposition method for manufacturing a thin film with a high mechanical strength such as an adhesion, wear resistance, solvent resistance, and the like without heating a substrate to a high temperature.
2. Related Background Art
Upon manufacture of a thin film such as an optical thin film, it is a general practice to heat a substrate during film formation to increase the mechanical strength such as an adhesion, wear resistance, solvent resistance, and the like of the thin film. For example, when an anti-reflection film of MgF.sub.2 as a low refractive index material is formed on a glass substrate by a resistive heating method or an electron beam irradiation heating method, the glass substrate is heated to about 300.degree. C. to improve the adhesion, wear resistance, solvent resistance, and the like of a thin film. However, such a method of heating a substrate cannot be applied to a plastic substrate of, e.g., polymethyl methacrylate (PMMA) which causes a considerable thermal deformation when it is heated to 100.degree. C. or higher, and a quartz substrate which deteriorates due to liberation of fluorine when it is heated to 200.degree. C. or higher.
In place of heating a substrate, an ion assist method for applying energy by irradiating an ionized gas to evaporated particles, and a cluster ion beam method for clustering evaporated particles, and thereafter, ionizing the particles have been developed. However, with these methods, the adhesion of a thin film may be impaired in the case of a plastic substrate or the like since the ion energy considerably damages the substrate surface. Even when a film is formed on a glass substrate, a neutralizer for preventing the substrate surface from being charged is indispensable.
FIG. 1 shows a conventional cluster ion beam deposition apparatus. A sealed crucible 131 is arranged in a vacuum chamber 110 which is controlled to a predetermined vacuum pressure by a vacuum pump connected to an evacuation port 110a, a thin film material in the sealed crucible 131 is heated by a heating bombard filament 132, and a generated vapor is injected from a nozzle 131a of the sealed crucible 131 into a vacuum atmosphere to cluster the vapor form into a mass of atoms. An electron shower generated by an ionization filament 133 is irradiated onto the clustered vapor (cluster particles) to ionize at least some cluster particles. The ionized cluster particles are then accelerated by an acceleration electrode 134 to be irradiated onto a substrate W.sub.O.
The above-mentioned cluster ion beam deposition method is expected to have a merit of forming a thin film having a high packing since a vapor of a thin film material is attached to a substrate as cluster particles in which a large number of atoms (normally, 1,000 atoms or more) are loosely bonded to each other. However, the surface of the substrate is damaged by bombardment of ionized cluster particles, and the optical characteristics or the like of a thin film may be impaired.
When a compound thin film is formed by supplying a reactive gas such as oxygen during film formation based on the cluster ion beam deposition method, the ionization filament for-ionizing cluster particles may deteriorate upon contact with the reactive gas. For this reason, in order to reduce contact between the reactive gas and the ionization filament by giving a directivity to the flow of reactive gas to be supplied into the vacuum chamber, the following means are proposed.
(1) means provided with a nozzle for injecting a reactive gas toward a substrate (see Japanese Laid-Open Patent Application No. 60-100668);
(2) means which uses both a nozzle for injecting a reactive gas toward a substrate, and a device for ionizing the reactive gas (see Japanese Laid-Open Patent Application No. 60-100669);
(3) means for ionizing a reactive gas by an ion source and supplying the ionized reactive gas to a vacuum chamber (see Japanese Laid-Open Patent Application No. 60-262963);
(4) means for ionizing and accelerating a reactive gas by an ionization filament together with cluster particles (see Japanese Laid-Open Patent Application No. 62-260053);
(5) means for ionizing and accelerating a reactive gas by glow discharge in a vacuum chamber (see Japanese Laid-Open Patent Application No. 63-011662);
(6) means provided with a discharging portion for exciting and activating a reactive gas in a vacuum chamber, and an orifice for injecting the activated reactive gas toward a substrate (see Japanese Laid-Open Patent Application No. 63-029925);
(7) means for ionizing and accelerating a reactive gas using an ionization device, and irradiating the ionized reactive gas onto a substrate by controlling (mass-separating or decelerating) the energy of the ionized reactive gas (see Japanese Laid-Open Patent Application No. 63-230868);
(8) means provided with a device for exciting, dissociating, or ionizing a reactive gas or a gas mixture of the reactive gas and an inert gas, a specific element, or the like near a substrate (see Japanese Laid-Open Patent Application No. 63-235468); and
(9) means for converting a reactive gas into excited neutral atoms (into ozone or gas in an atomic state) by silent-discharging the reactive gas (oxygen) by a discharging tube applied with a high-frequency voltage (see Japanese Laid-Open Patent Application No. 1-139758).
Also, an ion assist method for ionizing and accelerating an inert gas by an ion source to further improve the adhesion, hardness, and the like of a thin film formed by the cluster ion beam deposition method has been developed (see Japanese Laid-Open Patent Application No. 2-104661).
However, with the above-mentioned prior arts, as described above, it is expected to have a merit of obtaining a thin film with a high packing by clustering a vapor of a thin film material, but the surface of a substrate is damaged by high-speed bombardment of ionized cluster particles on the substrate, and the optical characteristics or the like of a thin film may be impaired. When a compound thin film is formed by supplying a reactive gas, it is difficult to sufficiently prevent an ionization filament for ionizing cluster particles from deteriorating even by giving a directivity to the gas by injecting the gas from a nozzle toward a substrate, as described above. Also, when the reactive gas is ionized and accelerated by the ionization device or ion source, since the ion energy of the reactive gas is added to that of ionized cluster particles, film quality is considerably impaired by abnormal discharging on the substrate surface or charge-up (charging) of a thin film during film formation. This problem cannot be satisfactorily solved even by arranging a neutralizer for neutralizing ions near the ionization filament for ionizing cluster particles, or the ionization device or ion source for a reactive gas. Furthermore, when a so-called ion assist method for improving the adhesion or packing of a thin film by irradiating an ionized inert gas onto a substrate together with ionized cluster particles is adopted, abnormal discharging on the substrate surface and charge-up of a thin film during film formation also pose problems as in the above-mentioned methods.
In addition, the ionization device or ion source for a reactive gas, a nozzle and an orifice for injecting a reactive gas toward a substrate, and the like make the film formation apparatus complicated, and as a result, the manufacturing cost of a thin film increases.
According to the prior arts, as described above, the method of improving the adhesion, wear resistance, solvent resistance, and the like of a thin film by heating a substrate to a high temperature cannot be applied to a substrate such as a plastic substrate which causes a deformation or deterioration due to heat. On the other hand, the ion assist method or the cluster ion beam method cannot be applied to a plastic substrate, and requires a neutralizer even for a glass substrate, resulting in a complicated apparatus.